In semiconductor technology photolithography is applied to structure a photoresist layer to be used as a mask. The photoresist layer is irradiated by a light source through openings in an opaque mask, which are shaped according to the pattern that is to be transferred to the photoresist layer. The photoresist is then treated with a developer solution. When a positive photoresist is used, the developer dissolves and removes the regions of the photoresist that have been exposed to light. When a negative photoresist is used, the developer dissolves and removes the regions of the photoresist that have not been exposed to light.
The absorption of incident light by a layer of negative photoresist decreases exponentially with the distance from the layer surface and does practically not change during the illumination. As the solubility of the negative photoresist is reduced according to the dose of radiation absorbed, the illumination yields a vertical profile of the solubility, which increases with increasing distance from the layer surface. The developer that is subsequently applied dissolves the entire region of the negative photoresist that has not been illuminated together with a lower layer portion of the illuminated region, whereas the photoresist is not dissolved at the surface of the illuminated region. An undercut is thus produced in the sidewall of the remaining photoresist structure. Negative photoresist is therefore especially suitable to produce structures with overhanging upper edges and with a reentrant sidewall profile.
The shape of the undercut depends on parameters of the developing process, in particular its duration and the temperature and amount of the developer, which can be controlled during the process. A minimal developing time is required to remove the photoresist completely from the area that has not been illuminated, which is the reason for a minimal size of the sidewall undercut. Furthermore, the developing progress is different for dense and isolated structures, and the sizes of the respective undercuts will differ correspondingly. Another problem arises from the fact that the insoluble upper layer portion rests on a relatively soft and thermally instable lower layer portion of the photoresist, which may later cause a degradation of the intended pattern under elevated temperatures, unless the whole photoresist layer is stabilized by a flood exposure.
Lift-off methods are applied to produce a structured layer, especially a structured metal layer, on a surface of a semiconductor device. Regions of the surface that are not to be occupied by the structured layer are covered with a sacrificial layer. An entire metal layer is applied on the bare areas of the surface and on the sacrificial layer, which is subsequently removed together with the overlying portion of the metal. After this lift-off, the remaining metal forms the structured layer. For this method to be feasible, the portion of the metal overlying the sacrificial layer must be separable from the rest of the metal layer. This is facilitated by an overhanging flank with a sharp upper edge of the sacrificial layer, so that the undercut sidewall of the sacrificial layer has a reentrant profile with negative slope. A conformal deposition of the metal is prevented by the sharp edge, where the metal layer is interrupted.
U.S. Pat. No. 4,024,293 A discloses a high sensitivity electron resist system for lift-off metallization. Resist films are formed by at least two layers of different radiation degradable organic polymers, which are developable by different solvents. One of the layers comprises a co-polymer of polymethyl methacrylate (PMMA) and methacrylic acid (MAA). The other layer is pure polymethyl methacrylate. The developer used for one of the layers does not attack the other layer even in areas that have been exposed to high-energy radiation, and vice versa.
U.S. Pat. No. 4,104,070 A discloses a method of making a negative photoresist image. A photoresist layer containing 1-hydroxyethyl-2-alkyl-imidazoline is exposed to actinic radiation according to the pattern to be produced. The photoresist is heated and subjected to a blanket exposure to actinic radiation. The portion of the photoresist that had not already been exposed previously is then removed with a solvent to yield a negative image.
U.S. Pat. No. 4,564,584 A discloses a photoresist lift-off process for fabricating self-aligned structures in semiconductor devices. A layer of a negative photoresist is applied on a structured layer of a positive photoresist. The entire structure is exposed to light, and the negative photoresist is thus rendered insoluble. Since the upper photoresist layer is at least translucent to the light, the remaining portions of the positive photoresist become soluble. They are removed together with the overlying portions of the negative photoresist. The remaining portions of the negative photoresist form a structured photoresist mask. This method provides a complete, mutually self-aligned image reversal of the patterns of the first and second photoresist layers.
U.S. Pat. No. 5,654,128 A discloses a single resist layer lift-off process for forming patterned layers on a substrate. The extent of the penetration of chlorobenzene into the resist layer is controlled by a post-soak bake. A post-metallization bake is employed to improve lift-off. The resist profile is provided with an increased overhang length and a negative slope of the sidewalls in order to prevent the sidewalls of the resist from being metallized and to facilitate the removal of the resist by lift-off.
U.S. Pat. No. 7,141,338 B2 discloses sub-resolution assist features (SRAF). In an image printed on a substrate, corner rounding and image shortening is reduced by illuminating a photolithographic mask and projecting light transmitted through the mask onto the substrate using an optical projection system. The mask has a pattern that includes at least one printable feature having at least one corner. A line feature is incorporated in the mask pattern in close proximity to a corresponding corner of the printable feature and has a line width that is smaller than a minimum resolution of the optical projection system.
U.S. Pat. No. 7,694,269 B2 discloses a method for selectively positioning sub-resolution assist features (SRAF) in a photomask pattern for an interconnect. The method comprises determining if a first interconnect pattern option, which is designed to be formed with SRAF, will yield a better circuit performance than a second interconnect pattern option, which is designed to be formed without SRAF. If the first option is preferred, one or more SRAF patterns are positioned to facilitate patterning. Otherwise the second option is selected as a target pattern and no SRAF is provided.